Voice communication is evolving from circuit-switched (CS) technology, such as provided by the Global System for Mobile communications (GSM), to packet-switched technology, such as provided by Voice over Internet Protocol (VoIP) techniques across Internet Protocol (IP) Networks. While voice telecommunication is evolving, wireless networks are evolving from circuit switched voice networks (e.g., GSM and Interim Standards 95 (IS-95)) to packet switched networks (e.g., Wireless Local Area Network (WLAN), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (cdma2000)), which are capable of supporting multimedia applications to mobile users over IP. Third Generation Partnership Project (3GPP) has specified the IMS in UMTS to accomplish the control and service functions of wireless IP multimedia. In this regard, 3GPP has adopted Session Initiation Protocol (SIP) as the signaling protocol in an IP multimedia subsystem (IMS). At the same time, in cdma2000, Third Generation Partnership Project 2 (3GPP2) has been developing the IMS, formerly referred to as the IP Multimedia Domain (MMD), to implement the control and service functions of wireless IP multimedia. 3GPP2 has also adopted SIP for use in the IMS specification. IMS was introduced, first, to handle traditional voice services over IP, second, to handle all multimedia services provided to subscribers. A UE can communicate with an IMS via a WLAN and gain access to IMS services, such as VoIP, The network, which provides a communication between a UE and an IMS, is referred to as an IMS domain in this specification. The network, in which voice communication is implemented with CS technology, is referred to as CS domain in this specification.
The current 3GPP Release 7 supports Voice Call Continuity (VCC) which makes it possible to hand over a call when a UE is to be transferred from an IMS domain to a CS domain or from a CS domain to an IMS domain. The UE provides integrated control over different radio access so that all the calls can be handled using appropriate radio access. The current 3GPP specification provides for a voice call continuity application server (VCC AS) that serves as an anchor point for a voice call (i.e., it is the node from which a handover is initiated) and controls and handles voice calls to and from the UE, regardless of whether of the same or different networks.
VCC provides for handing over a call between networks without voice disruption when a call is on-going (i.e., during call communication). Unfortunately, a handover during a period of ringing tone (the UE is a called terminal) or ring-back tone (the UE is a calling terminal) is not provided by the current 3GPP. A called UE will initiate a calling flow when the called UE is handed over from a CS domain to an IMS domain or from an IMS domain to a CS domain. Therefore, an Answer message to indicate that the phone is picked up by the user can't be transferred by the called UE. Because the Answer message can't be received by the calling UE, the call can't be accomplished. When a calling UE initiates a call in CS domain, a ring-back tone is provided by a Media Gateway (MGW) which is responsible for switching the traffic between the CS and IMS domain and for converting the voice call stream to a suitable form so that the voice call transfer between the CS and IMS domain is possible. The MGW is detached from the voice call path when the calling UE is handed over from the CS domain to the IMS domain. Therefore, if a handover is initiated during a call attempt (a period of call negotiation, ringing, or ring-back tone), the ring-back tone will not be received by the calling UE (the MGW is detached).
What is needed, therefore, is a method and system for controlling a handover during a call attempt (a period of ringing and ring-back tone) when the UE is to be transferred between a CS domain and an IMS domain.